1. Field of the Invention
This invention relates to modified aqueous dispersions of water-insoluble latex polymer and to compositions prepared using these dispersions. The modified latex polymer yields films which are useful as binders for coatings for leather and polyurethane foam and other substrates, and which show improved toughness, hardness and tensile strength while retaining substantial extensibility. The modified latex may be processed to yield cellular polymeric foams or coagulated to yield elastomeric gumstock which may be molded into articles or extruded as sheet for roll roofing membrane, protective warp and related applications. Thus this invention also relates to the synthetic polymeric coatings art, particularly the elastomeric and semi-elastomeric coatings arts and also to the thermoplastic elastomer art.
2. Brief Description of the Prior Art
Protective and decorative coatings for flexible or extensible substrates pose several serious challenges for the coatings chemist. For example, coatings for leather used in shoe uppers must stand up to repeated flexing without loss of adhesion or cracking. Thus, the coatings themselves must be relatively flexible. Coatings applied to many surfaces exposed to the elements must be able to withstand severe mechanical stress, as in the case of painted wood, the surface of which may be subjected to rapid and severe temperature changes during exterior exposure. Failure of the paint film in this case often appears as grain cracking and rupture of the film. Formed-in-place polyurethane foam roofs present an extreme example of the same phenomena. While hard (high glass transition temperature) polymeric binders may be used to prepare paints for wood trim and many other exterior applications, relatively soft (low glass transition temperature) binders are preferred for monolithic polyurethane foam roofing which encounters substantial thermal stresses.
In developing coatings for both leather and exterior polyurethane foam, the coatings chemist encounters the problem of preparing polymeric binders which are both flexible and durable. A related problem has been that low glass transition temperature polymers, for example acrylic polymers, are not only soft and flexible, but they also tend to be tacky. While the tack of these materials is often viewed as a virtue, as by the formulator of adhesives, in coatings applications tack is generally not desirable for both esthetic and functional reasons.
Further, many applications in which protective coatings are used, such as coatings for roofs, demand the high level of performance traditionally associated with solvent-based coatings and especially those based on thermosetting polymers. Yet, in these same applications often only coatings based on thermoplastic binders, such as employed in many water-based coatings, may be used because of a variety of practical problems.
In addition, environmental constraints mandate that the coatings chemist minimize the amount of organic solvent present in his formulation, a goal realizable by employing an aqueous dispersion of latex polymer as binder.
However, the unique properties of aqueous dispersions of latex polymer present problems which must be surmounted in order to approximate the performance of coatings based on polymers dissolved in organic solvents. It is conventional wisdom that the properties of coatings formed from aqueous dispersions of latex polymer should ideally reflect the nature and relative proportions of the comonomers used in preparing the polymer and be independent of the polymerization method used.
For example, an emulsion-polymerized latex which is essentially a homopolymer of a higher alkyl acrylate, such as 2-ethylhexyl acrylate (2-EHA), may, under the right circumstances, yield a coating film which has a glass transition temperature (T.sub.g) and other physical properties approximating that of a 2-EHA homopolymer prepared by bulk or solution polymerization. Meeting this ideal depends on a number of factors, including successful fusion of the individual latex particles to achieve a continuous film.
Latex particle fusion depends on interparticle diffusion of individual polymer chains. "Soft" particles composed of polymer molecules with glass transition temperatures significantly below ambient are known to fuse readily. Restrictions on chain diffusion, such as intraparticle crosslinking, tend to interfere with the chain diffusion process and reduce film fusion. When the monomer used to prepare the latex is sufficiently "hard" (i.e., corresponding homopolymers have relatively high T.sub.g), as in the case of polystyrene latex, no film formation takes place on drying the latex. In order to obtain "hard" coatings from water-dispersed latex, the coatings chemist has a variety of techniques at his command. For example, he may soften relatively high T.sub.g latex particles by swelling them with a fugative plasticizer, that is, a coalescent, which eventually evaporates from the dried film after formation. This will result in a "harder" (higher T.sub.g) film than could otherwise be formed. Alternatively, the chemist may crosslink the film after formation. At high crosslink density, measured film hardness may be significantly increased. On the other hand, a low level of post-film formation crosslinking will enhance the elastomeric properties of the film.
In some coatings applications, such as the area of protective coatings for monolithic, spray-in-place polyurethane foam roofing, a relatively soft coating is desired, so that the coating may conform with the thermal expansion and contraction of the substrate. On the other hand, it is also desirable that this kind of coating be tough, and to some extent, elastomeric. Coated roofs must often be walked upon to obtain access to roof-mounted ventilators, air conditioning heat exchangers, skylights and the like. Similarly, they should resist mechanical damage from hailstones and the effect of the aggregate which is sometimes used to protect the surface. Prior to the present invention, toughness could be imparted to soft protective coatings through crosslinking the polymer chains or by the addition of reinforcing fillers.
Polymer films may be crosslinked by a variety of techniques. For examples, the polymer chemist may include comonomers in a latex polymer which have two or more sites of functionality with different reactivities with the intention of crosslinking the film after fusion of the latex parties.
For example, one site may be that of ethylenic unsaturation so that the monomer will copolymerize, and the other may be a halogen or other reactive moiety, so that the polymer chains may be crosslinked after film formation. This technique is employed in the case of acrylic elastomers, where both vinyl chloracetate and 2-chloroethyl vinyl ether are used as comonomers with ethyl acrylate to prepare latex particles by emulsion polymerization. The latex particles of elastomer are coagulated, dried and molded to the shape of the desired article. Subsequently, the elastomer is vulcanized by heat-activated crosslinking of reactive halogen sites through a crosslinking or vulcanization agent such as sodium stearate/sulfur. Note that if the latex particles or coagulum were crosslinked prior to molding, it is unlikely that the desired article could be successfully molded, as crosslinking "fixes" the shape of the particles or coagulum by restraining the migration of polymer chains necessary to achieve a continuum within the molded article. The particles are no longer plastic. The same deficiency is observed in crosslinked latex particles intended for surface coatings applications. Although a small degree of crosslinking, desirable for such reasons as reduced particle swelling, may not have a severe adverse effect on film formation, heavily crosslinked particles may not form films, even though the constituent monomers are appropriately soft by the Tg criterion, because the crosslinking interferes with the inter-particle polymer diffusion necessary for good film formation.
In the case of natural rubber latex, which contains a large proportion of ethylenic unsaturation in the polymer chain backbone itself, and in the case of chemically similar synthetic rubber latexes, the latex may be crosslinked to a substantial degree prior to coagulation and molding. This "prevulcanization" may be effected by using ionizing radiation as a free radical source, and it may be sensitized by swelling the rubber latex with monomer containing multiple sites of ethylenic unsaturation. Because of the very high proportion of potential crosslinking sites on the rubber polymer chain, and the low Tg of the polymer, a substantial degree of prevulcanization may occur without severely affecting the ability of the latex coagulum to flow sufficiently at the elevated molding temperature to produce the shape of the desired article. Inter-particle polymer diffusion is, nevertheless, inhibited to some extent by prevulcanization, and the properties of prevulcanized rubber are not equivalent to those of post-vulcanized rubber. When there are only a few crosslinking sites distributed randomly along the polymer chains in a latex particle, crosslinking necessarily imposes long range constraints on the diffusive freedom such molecules would otherwise enjoy.
In many coating applications it is either undesirable or impossible to post crosslink films formed from latex particles, although the properties of such films would be substantially enhanced by crosslinking. For example, a coating may be applied in the field over exterior substrates such as concrete buildings and other structures, and it may be impossible to apply heat to the coated substrate in a controlled fashion sufficient to activate conventional crosslinking agents. Crosslinking agents reactive at ambient temperature present a different host of problems such as high toxicity, flammability and long term residual environmental persistance. Finally, the additional skilled labor required to apply the crosslinker may simply be unavailable or unjustifiable economically. Yet the coatings chemist is severely hindered if he is restricted to a palette consisting of comonomers of differing T.sub.g in his effort to paint a latex coating which is both tough and extensible. There is a need for coating compositions which offer the convenience and low toxicity of latex polymer aqueous dispersions with the enhanced toughness and reduced tack of post-crosslinked systems.
Latex of relatively low T.sub.g thermoplastic polymers may be coagulated in batch or continuously to yield elastomeric gumstock or pellets which may be further processed and/or modified as is typical of elastomeric materials. Alternatively, the latex coagulate itself may be modified by addition of crosslinking agent, fillers and/or processing aids and subsequently molded to give shaped articles such as gaskets, "O" rings, shoe heels and the like. The gumstock or coagulate may also be further processed as sheet which may be reinforced by the incorporation of fibrous material such as chopped glass, glass scrim, fiber roving and the like. The elastomer properties of coagulated low T.sub.g latex polymers are generally improved by the addition of crosslink agent during processing. However, addition of such agent may require additional processing time and increased capital and materials costs for the manufacturers of elastomeric articles. While "prevulcanized" natural and synthetic rubber latex polymers are available to the processor, and such materials requiring either a reduced amount of crosslinking agent during processing or none at all, there is a need for latex polymer which contains essentially no sites of ethylenic unsaturation, in contrast to "prevulcanized" rubber, and increased toughness while requiring the addition of little or no external crosslinker during processing of the coagulum.
Further, in many applications, especially coatings, semi-elastomeric materials are preferred to true elastomers, because the ability to dissipate applied stress by flow over a relatively long time period is a virtue. However, there is a need for materials which not only can slowly dissipate suddenly applied stresses by flow, but also which are not tacky at ambient temperatures as are many commercial thermoplastic elastomers.